The invention relates to a pencil sheath, and to a composition therefor adapted particularly for sheaths for cores of graphite and similar rods for the manufacture of pencils.
It is known that in addition to pencils manufactured by the classical method with wooden sheaths, pencils with sheaths made of different compositions of plastic materials are used. Such plastic sheaths incorporate foaming agents in order to improve their cutting properties. A drawback of pencils manufactured with already known sheath compositions is their relatively low bending strength and frequently a not quite uniform structure of extruded pencils.
Another drawback of previous actually used compositions is also a not always suitable combination of the requirement for good cutting properties when sharpening pencils with an extruded profile and with sufficient bending strength, and furthermore the not always perfect adhesion of the sheath with graphite, together with the possibility of their quick and cheap manufacture.
An increase of bending strength has been obtained in prior pencils by the addition of suitable fillers, as for instance sawdust, cut glass fibers, and the like, and in addition in the course of manufacture of the required profile by the connection of a number of strands and the like. However, the results have not been always satisfactory.